1. Field of the Invention
The present invention relates generally to telecommunications and more particularly to improving security and data transmission over fiber optic networks.
2. Background Information
In current fiber optic networks, an electronic data stream is fed to a laser amplitude modulator. The laser amplitude modulator typically pulses or alters the laser output to create an amplitude-modulated optical signal representative of the electronic data stream. The laser amplitude modulator and laser thus define a transmitter for transmitting the optical signal over an optical fiber, which is then received by a receiver. The receiver for the amplitude-modulated optical signals of the optical data typically includes a photodiode to convert the optical signals back into the electronic data stream.
The reading of the amplitude-modulated optical data signals using a photodiode is straightforward: the optical signals either produce an electric output at the photodiode or they do not. As a result, an output electronic data stream of zeros and ones is generated.
However, optical fiber may be tapped. The optical fibers can be spliced or even merely clamped so as to obtain optical signals from the fiber. It also may be possible to tap fibers without physically touching the optical fiber, for example by reading energy emanating or dissipating along the fiber. Amplitude-modulated optical signals, with their ease of detection from a photodiode, require that only a small amount of energy be tapped and passed through the photodiode in order to be converted into a tapped electronic data stream.
To confront non-secure optical and non-optical data lines, it has been known to use public key/private key encryption so that the data stream being transmitted is encoded in a format that makes it difficult to decode. Encryption however has several drawbacks, including the need for extra processing steps and time. Moreover, public key/private key encrypted data can be cracked, and the devices and algorithms for doing so are constantly improving.
U.S. Pat. No. 5,455,698 purports to disclose a secure fiber optic communications system based on the principles of a Sagnac interferometer. A data transmitter is a phase modulator for modulating counter-propagating light beams sent by a receiver round a loop. The receiver includes a light source, a beamsplitter for splitting light from the light source into counter-propagating light beams and for receiving the phase-modulated light beams, and an output detector. U.S. Pat. No. 5,223,967 describes a similar Sagnac-interferometer-based system operating over a single optical fiber.
The Sagnac-interferometer-based systems described in these patents have the disadvantage that they require the light to travel over a loop, whether back and forth in a single fiber or over a long length looped fiber. As a result, either the link budget for the single fiber must be doubled, reducing the data carrying capacity for a single fiber, or else a looped fiber with significant and expensive extra length of at least twice that of a single fiber must be laid between the transmitter and the receiver. Moreover, the receiver contains the light source, as opposed to the current installed base where the transmitter has the light source.
The Sagnac-interferometer-based systems thus are expensive to build and operate, and do not work particularly well with existing systems.
U.S. Pat. No. 6,072,615 purports to describe a method for generating a return-to-zero optical pulses using a phase modulator and optical filter. The RZ-pulse optical signal transmitted over the fiber is easily readable by a detector.
U.S. Pat. No. 5,606,446 purports to describe an optical telecommunications system employing multiple phase-compensated optical signals. Multiple interferometric systems are combined for the purpose of multiplexing various payloads on the same optical transmission path. The patent attempts to describe a method for providing fiber usage diversity using optical coherence length properties and a complex transmit/receive system. Each transmitter has a splitter, a plurality of fibers and a plurality of phase modulators to create the multiplexed signal, which is then demultiplexed at the receiver. This system is complex and expensive.
An object of the present invention is to provide an improved security optical fiber transmission system and device. An alternate or additional object of the present invention is to provide high bandwidth optical data transport via transmission and recovery of phase-modulated optical signals. Yet another alternate or additional object of the present invention is to provide a simple yet secure phase-modulated optical data transmission system.
The present invention provides a fiber optic data transmission system comprising a transmitter having a light source, a phase modulator for phase modulating the light source and a controller for the phase modulator. The controller controls the phase modulator as a function of an input electronic data stream and a second electronic data stream employing a delay, so as to create an encoded phase-modulated optical signal in the light passing through the phase modulator. The system also includes an optical fiber receiving the optical signal and a receiver receiving the optical signal from the optical fiber. The receiver includes a splitter for splitting the optical signal into a first path and a second path. The second path has a second path length longer than the first path length, the second path length being a function of the delay in the second electronic data stream. The receiver also includes a coupler for coupling the first path and the second path together so as to create an optical output signal.
With the system of the present invention, the receiver functions as an interferometer. An attempt to read the optical signal in the fiber, for example from a tap, requires knowledge of the delay and the creation of a precise physical delay path in the interferometer. Optical detectors with photodiodes do not have the bandwidth to measure the phase-modulated optical signal directly, since photodiodes are only capable of determining whether or not light is present.
The interferometer of the receiver of the present invention requires a significant amount of light to pass through the splitter and coupler, so that a tap would have to remove a significant amount of energy from the optical fiber in order to resolve the optical signal without a significant bit error rate. Detection of a tap on the system of the present invention, for example through a detection device reading a level of light energy in the fiber, becomes almost certain.
Moreover, the tap would have to match the interferometer delay in the second path to the electronic delay imposed by the controller, which is not always known.
The controller preferably includes an exclusive-or gate function, the input data stream being fed to an input of the exclusive-or gate and the second data stream being an input of the exclusive-or gate and a function of the output of the exclusive-or gate. The second data stream thus runs in a feedback circuit, which preferably includes a delay circuit delaying the second data stream by an amount of time directly proportional to the bit rate. As such, the controller may comprise a delayed-feedback exclusive-or gate.
The delay circuit may delay the second data stream by an amount of time directly proportional to a predetermined number of bits. When the data rates are at 155 Mb/sec (OC-3) or higher, the amount of delay preferably is greater than one bit. However, for data rates below OC-3, the electronic delay can be a fraction of the bit time, as long as the fraction is a power of two, for example one-half, one-quarter, one-eighth, etc. The fractional delay permits the present system to run relatively low data rates, such as T1, without requiring a long coherence length source.
The light source preferably is a continuous wave laser, for example a semiconductor laser operating at approximately 1550 nm or other wavelengths. The transmitter of the present invention requires only one phase modulator, and can operate at speeds of up to 10 Gb/s and even faster.
The receiver may include a detector for converting the output optical signal into an electronic output data stream. Preferably, the path length difference between the first path length and second path length is a function of the delay and the speed of the light in the fiber. The distance delays the light traveling in the second path with respect to light in the first path by a second delay, the second delay preferably being equal to the delay imposed at the controller. The second delay may vary slighty from the first delay, as long as the detector at the detector can read the output signal. The actual permissible difference will depend on the light source and any electronic filtering of the output signal.
The system preferably includes a detector for detecting a tap or loss of energy in the optical fiber. Most preferably, the detector is an energy sensor, which may or may not include programmable xe2x80x9ctripxe2x80x9d levels, which can monitor the amplitude of the light in the fiber. If a tap occurs, it must couple off a significant amount of energy to pass through an interferometer with a low bit error rate, thus making detection of the tap by the detector highly likely.
Depolarizers preferably are located between the light source and the phase modulator, and in the first path of the receiver. The depolarizer in the receiver alternatively may be in the second path.
The present invention also provides a transmitter comprising a light source, a phase modulator for phase modulating the light source, and a controller controlling the phase modulator, the controller including a delayed-feedback exclusive-or gate.
In addition, the present invention also provides a receiver comprising an interferometer, the interferometer having a first path and a second path propogating light at a delay with respect to the first path, the delay being a function of a delay imposed by a phase-modulator controller in a light-emitting transmitter.
A method for transmitting secure data is also provided comprising the steps of:
transmitting light from a light source in a transmitter;
electronically imposing a delay on an electronic data stream; and
phase modulating the light in the transmitter as a function of the selected delay and an electronic data input stream.
Preferably, the phase modulated data is a function of an output of a delayed-feedback exclusive-or gate.
The method further may include receiving optical signals in a receiver, splitting the optical signals into a first and second path and imposing a second delay on light in the second path with respect to light in the first path. The second delay is a function of the electronically-imposed delay and most preferably is equal to the electronically-imposed delay. The first and second paths then are recombined so as to generate an output optical signal, which may be read by an optoelectronic detector.
The method preferably includes monitoring a fiber for intrusion. The monitoring preferably includes monitoring an energy level in the fiber with programmable trip levels.